This invention relates to a hydraulic control system for a four-wheel drive torque transfer mechanism for a vehicle which is coupled with an automatic transmission for switching the mode of operation between high and low positions and between two-wheel drive and four-wheel drive.
Generally, a four-wheel drive torque transfer mechanism has the gear positions (ranges) of a two-wheel drive direct-coupling mode (hereinafter referred to simply as "H.sub.2 "for brevity), a four-wheel drive direct-coupling mode (hereinafter referred to simply as "H.sub.4 " for brevity), and reduced-speed four-wheel drive mode (hereinafter referred to simply as "L.sub.4 " for brevity). In order to establish the respective operation modes of H.sub.2, H.sub.4 and L.sub.4, the four-wheel drive torque transfer mechanism which is coupled with an automatic transmission is provided with selectively controlled frictionally engaging and disengaging members in the form of hydrauically operated wet multiple disc friction clutches and brakes to effect gear shifts under control of a hydraulic control system which utilizes the hydraulic pressure of a control circuit for the automatic transmission itself.
In order to prevent dangerous engine overrun and vehicle wheel locking upon shift to L.sub.4 range during a high speed operation in H.sub.2 or H.sub.4 range, the hydraulic control system of the conventional four-wheel drive transfer mechanism which is connected to an automatic transmission is usually provided with an inhibitor valve in a conduit leading to a frictional engagement means which establishes the L.sub.4 range, thereby turning on and off the conduit according to the level of the governor pressure drawn from the hydraulic control circuit of the automatic transmission. With such an arrangement, when the L.sub.4 range is selected by a manual shift device, the inhibitor valve blocks the conduit if the governor pressure which is commensurate with the vehicle speed is at a high level, inhibiting a change of the transfer mechanism to L.sub.4 range and permitting a change to L.sub.4 range only after the governor pressure or the vehicle speed has dropped below a predetermined level, by engaging the frictional engagement means for the L.sub.4 range. Once changed into the L.sub.4 range, the vehicle operation remains in that range even if the governor pressure exceeds the predetermined level until the range of H.sub.2 or H.sub.4 is selected by the manual shift device.
However, the automatic transmissions are generally arranged to stop the output of governor pressure when the manual valve or manual shift lever is set in N (neutral) range and P (parking) range positions. Therefore, for example, if the L.sub.4 range is selected by a manual shift device of the torque transfer mechanism and at the same time the manual valve of the automatic transmission is set in the N-range position during operation in the range of H.sub.2 or H.sub.4 of the transfer mechanism at a speed above a predetermined value, the absence of the governor pressure creates a situation as if the vehicle speed were substantially zero. Consequently, the conduit under the control of the above-mentioned inhibitor valve is turned on to set the four-wheel torque transfer mechanism in the gear position of L.sub.4 range. Therefore, under these circumstances, if the manual valve is shifted to the D (drive) range position, a shift to L.sub.4 range of the transfer mechanism is effected even in a high speed operation, causing engine overrun and locking of vehicle wheels which may lead to a dangerous slip accident.